The briefing, organized by The American
Chemical Society and the American Institute of Chemical Engineers, was opened
by Senator Lamar Alexander (R-TN), a member of the Senate Committee on
Environment and Public Works. The briefing was hosted by the U.S. Senate's
Science and Technology Caucus.

Oil dispersants work like dishwashing
detergent, breaking oil into droplets that mix more easily with the surrounding
water. They are designed to break up surface oil slicks, thereby reducing
damage to the shoreline; protecting animals such as sea turtles that spend extensive
time at the ocean surface; and increasing the rate at which microbes can consume
the hydrocarbons. To date, BP has released approximately 2 million gallons of
dispersants into the Gulf.

Diaz, an ecologist with expertise in
bottom-dwelling communities and low-oxygen marine "dead zones," briefed members
of the Congress and their staff on the ecological trade-offs inherent in
dispersant use.

"Applying dispersants represents a
decision to increase the hydrocarbon load on one part of the ecosystem while
reducing the load on another," said Diaz. "It decreases risk at the surface and
shoreline while increasing potential risk to organisms in the water column and
on the seafloor."

Diaz stressed that assessing the
long-term risks and impacts of dispersant use will require study of the Gulf ecosystem
as a whole. He says "We now have small drops of oil dissolved throughout the
water column in large areas of the Gulf, so resident organisms can encounter
oil during different stages of their life cycle, and throughout the food chain."

As an example, Diaz noted that white,
pink, and brown shrimp-important targets of the Gulf fishing industry-all spawn
offshore, with juveniles subsequently moving into coastal nursery areas. He
said that crab larvae-important food for many other species-have already been
found with droplets of oil under their shell.

Diaz noted that migratory animals-including
Atlantic bluefin tuna-enter Gulf waters during sensitive parts of their life
cycle. "The Gulf of Mexico is an important spawning and nursery habitat for
these fish," says Diaz. "They're drawn by the Gulf's warm waters, abundant prey,
and Sargassum-a type of floating
seaweed that harbors an ecosystem of its own."

Diaz also noted that scientists'
understanding of the environmental effects of dispersants decreases with depth.
"We've done toxicity testing on shallow-water species that are easily grown or
kept," said Diaz, "so the risks that dispersants pose to surface species are
pretty well understood. But as we move from marshes to the deep-sea, our
knowledge declines sharply."

He cautioned that dispersed oil could reach
the bottom along with sinking sediment, potentially affecting the coldwater
coral ecosystems of the deep Gulf. These unique habitats, in depths exceeding
1,000 feet, feature a diverse community that includes coldwater corals such as Lophelia pertusa and Madrepora oculata.

During his briefing, Diaz also discussed
biodegradation-the process by which bacteria and other microorganisms
metabolize the various compounds in petroleum. Diaz noted that the
biodegradation depends on the type of dispersant used, its application method
and rate, the type of crude oil present, and the composition of the microbial
community.

He emphasized that breakdown of
dispersed oil depends on the availability of oxygen in both the water and
sediment. "There are two main low-oxygen zones in the Gulf," says Diaz.
"There's a seasonal ‘dead zone' on the continental shelf fueled by human input
of nutrients to the Mississippi watershed, and a naturally occurring ‘oxygen
minimum zone' that intersects the continental slope at depths from around 600
to 2,300 feet."

Diaz says there are two main questions
related to these low-oxygen zones: first, whether there's enough oxygen in the
water or sediments to fuel the metabolism needed to degrade the oil, and
second, to what degree biodegradation will further exacerbate the low-oxygen
zones. Scientists are actively pursuing both questions.

Because metabolic rates decrease with
both water temperature and depth, Diaz
says that the efficiency of oxygen depletion decreases with depth as
well. He estimates that microbes would require 187 barrels of oil per square
kilometer to lower oxygen levels by 10 per cent in the Gulf's surface waters (0
to 600 feet), 400 barrels/km2 for a 10 per cent oxygen reduction between
600 and 2,400 feet, and 650 barrels/km2 from 2,400 to 4,500 feet. His
estimates assume that the oil is uniformly distributed throughout the water
column.

The briefing was moderated by Deborah
Grubbe, chair of the Institute for Sustainability and president of Operations
and Safety Solutions, LLC. The other panelists were Kenneth Lee, executive director
of the Centre for Offshore Oil, Gas and Energy Research, Fisheries and Oceans
Canada; and Nancy Kinner, co-director of the Coastal Response Research Center
at the University of New Hampshire.

The briefing was part of the American
Chemical Society's ongoing Science &
the Congress program, which was established in 1995 to provide a neutral
and credible source of scientific information targeted to policymakers on
Capitol Hill. The program has offered more than 120 briefings to date.

Videos of the briefing will be
available on the ACS Science & the
Congress website in a few weeks.

Diaz first studied dispersants in the
late 1970s, when the U.S. Department of the Interior asked VIMS to conduct a
large-scale ecological study of Virginia's outer continental shelf in light of
interest in offshore domestic drilling following the 1973 oil crisis.